In recent years, the recording density of magnetic disk drives has been significantly improved. With the recording density of magnetic disk drives improved, the spacing between a magnetic head (more specifically, a read element and a write element of the magnetic head) and a recording surface of a magnetic disk has been increasingly reduced. For example, in a recently-emerged magnetic disk drive, the distance between the magnetic head (hereinafter referred to as the head) and the magnetic disk (hereinafter referred to as the disk), that is, the flying height of the head, is about 1 nanometer (nm).
The reduced spacing is likely to induce a high-fly write failure (hereinafter referred to as an HFW failure). The HFW failure refers to a phenomenon in which data is written by the head to the disk at a spacing larger than a normally set spacing. A cause of the increased spacing for the head is a collision of the head against a surface lubricant on the disk or grease spattered on the disk, leading to jump-up of the head.
When data is written by the head to the disk at an increased spacing, an error is likely to occur when the data is read (that is, a read error is likely to occur). A sector error rate (SER) is known as an index indicative of the rate at which read errors occur. The SER is measured for each of the heads arranged in association with the respective recording surfaces of the disks. The SER in this case is indicative of the rate at which read errors occur when data is read by the head from the corresponding recording surface of the disk in units of sectors. Furthermore, a magnetic disk drive in which the recording surface of the disk is partitioned into a plurality of zones for management may measure the SER for each combination of the head and the zone. The SER in this case is indicative of the rate at which read errors occur when data is read by the head from the corresponding zone of the disk in units of sectors.
The conventional magnetic disk drives adjust the format of the recording surface of the disk, for example, a linear recording density BPI (Bits Per Inch) for each head (or for each head and zone), so as to make the SERs corresponding to all the heads (or combinations of all the heads and all the zones) equivalent.
As described above, when data is written by the head to the disk at an increased spacing, an error is likely to occur when the data is read. For such a head, when the SER is measured with the flying height of the head increased, the SER is significantly degraded. Here, an index indicative of a change (degradation) in SER (read error rate) corresponding to a change in the flying height of the head (more specifically, a change by a unit length, for example, a change by 1 nm) is intended to be referred to as flying-dependent error sensitivity [SER/nm].
The flying-dependent error sensitivity [SER/nm] varies depending on the head. In connection with the flying-dependent error sensitivity [SER/nm], heads are roughly classified into a first type and a second type. The first type head has a high flying-dependent error sensitivity [SER/nm], and thus the SER of the first type head is significantly increased (that is, the SER is degraded) by a slight increase in flying height during write. The second type head has a low flying-dependent error sensitivity [SER/nm], and thus the SER of the second type head is not significantly increased by a slight increase in flying height during write.
However, in adjusting the format of the recording surface of the disk, the conventional art fails to take into account whether the head associated with the recording surface is of the first type in which the SER of the head is significantly increased by a slight increase in flying height during write. Thus, the conventional art has difficulty preventing the read error rate from being increased by an HFW failure if the head is of the first type.